Method of coating electrical devices



' Jan. 11, 1949. R. w. DE MONTE ETAL 2,459,013

METHOD OF COATING ELECTRICAL DEVICES Filed March 22, 1945 R. w DEMONTE 51 A. 8. HAINES A TTORNEY Patented Jan. 11, 1949 UNITED STATES PATENT ()FFICE METHOD OF COATING ELECTRICAL DEVICES Application March 22, 1945, Serial No. 584,064

('31. Ill-75) 4 Claims.

This invention relates to a coating process for producing a moisture-resistant coating on a metallic or non-metallic surface.

In its preferred embodiment the invention is particularly applicable to electrical devices such as transformers, retard coils and the like. In protecting such a device from the adverse eifects of moisture it has generally been customary to place the device within a hermetically sealed metal container filled with a suitable potting compound. However, there are many instances, such as in airborne or portable equipment, where the additional weight of the container and the potting compound and the resulting increase in size are objectionable.

It is an object of the present invention to provide such a device with an adherent moistureresistant coating which will maintain its characteristics over a wide temperature range.

In accordance with one form of this invention, the device, after being dried in a vacuum, is subjected to one or more vacuum impregnations with a polymerizable varnish, each impregnation being followed by an air drying and subsequent baking treatment to evaporate the solvent and substantially completely cure the polymerizable material. Thereafter the impregnated device has applied to it one or more coatings of a mixture of the said varnish and a suitable inert inorganic finely divided filler, each coating being baked for such a time and at such a temperature that the polymerizable material in the varnish is not completely cured but is left in a more flexible state.

The invention will be better understood by reference to the following detailed description and the accompanying drawing in which the figure represents one type of electrical device which is provided with a moisture-resistant coating in accordance with the process of the present invention.

The power transformer disclosed in the drawing is of the usual type comprising a closed laminated core I with the coil form 2 comprising the primary and secondary windings surrounding one leg of the core and with the coil 2 completely surrounded by a spiral wrapping 3 of an insulating tape of muslin, for example.

The present invention is concerned with the provision of a moisture-resistant coating for such a device, which coating will be substantially moisture-proof even when the device is subjected to water immersion and which will maintain its moisture-resistant properties over a wide temperature range, such as from -55 C. to +125 C.

The preferred procedure for applying a moisture-resistant coating to a transformer of the type disclosed in the drawing will now be described.

Impreynation After the transformer has been thoroughly dried by baking in a vacuum for several hours at a temperature of about 230 F'. the transformer is immediately placed in an unheated tank and maintained under a vacuum while cooling from the baking temperature. An impregnating compound, such as a polymerizable varnish, is then drawn into the tank to cover the transformer completely while still maintaining a vacuum in the tank. The vacuum in the tank is maintained for several minutes after which the vacuum is broken and the transformer retained immersed in the impregnating compound for a further period, forty;five minutes for example. The transformer is then removed from the tank, is air dried and drained at room temperature for about one hour and is then baked at a temperature of about 270 F. for about 8 to 10 hours, or until the polymerizable material in the impregnatin compound is substantially fully cured. The impregnated transformer is then allowed to cool in the evacuated tank after which the transformer is given a second impregnation with the same impregnating compound in accordance with the same procedure described for the first impregnation including, as a final step, baking at the same temperature and for the same length of time,

Coating The doubly impregnated transformer is then given two coatings of a uniform mixture of a coating compound, comprising a polymerizable varnish and finely divided talc (magnesium silicate) in the approximate proportions of 3.5 pounds of talc to 4.1 pounds of the resin content of the varnish.

The impregnated transformer is first completely immersed 'in the coating compound for a sub stantial period and for at least ten minutes after which the transformer is withdrawn, drained and air dried at room temperature for 5 or 6 hours, followed by slowly baking the coated transformer for a period of about 5 hours at a temperature ofabout 200 F. :10 F.

The coated transformer is again completely immersed in the coating compound for at least ten minutes after which it is withdrawn, drained and air dried at room temperature for 5 or 6 hours, followed by slowly baking the coated transformer for a period of about 11 to 12 hours at a temperature of about 200 F. i 10 F.

The period of air drying at substantially room temperature prior to the baking at an elevated temperature is desirable to facilitate the evaporation of the thinner or solvent of the varnish before the coating compound has become hardened. The relatively slow rate of partial curing each coating is desirable to complete the driving off of the volatile thinner at such a slow rate as to prevent the formation in the coating of Voids or holes which if present would result in reduced resistance of the structure to moisture penetration.

It is preferable that the same varnish be used as the impregnating compound and in the coating compound. One varnish which has been found quite satisfactory for both the impregnating compound and the coating compound is Harvel 612C varnish, manufactured by the Irvington Varnish and Insulator Company. This commercial varnish may be used unchanged in the coating compound but it will generally be found advisable to add a small amount of thinner, such as petroleum naphtha, to the varnish when it is to be used as the impregnating compound. This Harvel varnish comprises the condensation product of formaldehyde and the oil of the cashew nut shell (which contains a phenolic-like component) dissolved in a suitable solvent, such as petroleum naphtha, the varnish having a solid content of about 57 per cent i 2 per cent. However, other heat reactive polymerizable varnishes or other synthetic resinous coating materials having a low oxidation rate when exposed to air may be employed in the process of this invention.

Purified talc is the preferred finely divided inert inorganic filler to be added to the varnish to form the coating compound. The talc may be either fibrous or amorphous although the amorphus form is preferred. Good results have been obtained when the talc is such that it will pass through a 200 mesh screen. Talc is particularly advantageous because of its low temperature coefiicient of expansion, and because its relatively low density enables it to be uniformly suspended in the varnish. Tale is also desirable because of its good wetting characteristics in varnish and its poor wetting characteristics in water. However, other finely divided inert powders of low temperature coefiicient of expansion may be employed in place of talc, such as sand, mica, asbestos, aluminum silicate and chalk.

In order to achieve the best results in making the coated transformer moisture-resistant, it has been found that the amount of talc employed in the coating compound is somewhat critical. It is preferred that the varnish and the talc be mixed in such proportions that the weight of the talc is about 85 per cent of the weight of the solid content of the varnish. However, good results will be obtained if the weight of the talc lies within the range from 80 per cent to 90 per cent of the weight of the solid con-tent of the varnish.

I11 any event, the weight pf the talcused, in, the.

Finely divided asbestos 88 to 98 Finely divided aluminum silicate to 90 Finely divided chalk 88 to 98 It has also been found that the consistency of the varnish-filler coating mixture is somewhat critical in producing the best results. At the time the coating compound is ready for use, its preferred consistency is of the order of 49 seconds i 4. seconds when measured in an A. S. T. M. cup with a 0.20 inch hole in accordance with A. S. T. M. method 13-333 for testing clear lacquers and lacquer enamels as published in 1942 in part II of American Society of Testing Materials Standards. Part of the thinner in the varnish may be evaporated or additional thinner added to meet this requirement.

The number of impregnations and the number of coatings to be employed depend somewhat upon the size and configuration of the device to be protected and the temperature range to which the device may be subjected in service. With a small size transformer having over-all dimensions of 4 inches by 4 inches by 4 inches of the configuration shown in the drawing, it has been found that multiple impregnations and multiple coatings are desirable when the requirements are such that the device must maintain its moistureproofing characteristics over a wide temperature range, such as from 35 C. to C. If more than two coatings are found desirable, the final coating should be baked at the temperature and for the time interval specified above for the second coating, while all earlier coatings should be baked at a temperature and for a time interval the same as specified above in the first coating. For coils larger than the one assumed above, more than two coatings will. generally be desirable; even for small size coils more than two coatings may be found advantageous in many instances to obtain increased moisture-resistant characteristics.

An important feature of the baking treatments above-described is that while the varnish applied to the device in the two impregnating steps is fully cured, the varnish applied in the two coating steps is only partially cured; that is, the baking temperature and the duration of the balcing for each coating are such that the resin content of the coating is only partially polymerized whereby flexible coatings and not hard brittle coatings are produced. With flexible coatings produced in this manner, the device has been found to maintain its moisture-resistant characteristics even when subjected to such wide temperature variations that would cause a fully cured coating to crack.

In order to distinguish more clearly from the partial curing of the varnish-filler coating and the full curing of the varnish impregnations, it may be stated that for each varnish-filler coating the baking is for a time and a temperature such that the coating while solid is still thermoplastic, the resin has not reached thestage of infusibility and the resinous material is still soluble in its original solvent; while for the full curing in the impregnations the varnish is made infusible, is no longer thermoplastic and is insoluble in its original solvent.

After the transformer has had applied thereto two coatings of the varnish-filler mixture a subsequent inspection will disclose no line of demarcation between the first coating and the second coating. The two coatings appear as a single coating and it is impossible to strip off the second coating from the first coating. Since at the time the second. coating is applied the resin in the first coating is still in a partially soluble state, it is likely that the surface of the first coating is softened somewhat by the solvent in the second coating to coalesce the two coatings into a unitary coating as the result of the final baking.

It has also been found preferable to achieve the partial curing of the resin in the coating compound at a relatively slow rate, that is, by using a relatively low baking temperature for a considerable length of time rather than by baking at a higher temperature for a shorter time. In particular, for the first coating of varnish and talc the preferred baking is at a temperature of about 199 F. for a period of about 5 hours, although the same amount of partial curing may be obtained at a baking temperature of about 180 F. for about 8 hours or at a baking temperature of about 210 F. for about 4 hours. For the final coating of varnish and talc the preferred baking temperature is about 190 F. for a period of about 12 hours although the same amount of partial curing may be attained at a baking temperature of about 180 F. for a period of about hours or a baking temperature of about 210 F. for a period of about 10 hours.

It should also be noted that each coating applied prior to the final coating is baked for a much shorter length of time than the final coating; this is because the baking of the final coating also increases the amount of curing of the earlier coatings and the total baking to which an inner coating is subjected should not be suffi cient to fully cure the resin of the inner coating.

The transformer disclosed in the drawing illustrates the device after it has been subjected to two impregnations followed by two coatings as described earlier in this specification. It is assumed that the varnish applied in the first impregnation is almost entirely absorbed by the coil form 2 and the muslin tape 3, while forming a thin coating 4 on the metal core I. For the second impregnation it is assumed that not all of the varnish is absorbed by the coil but some of it forms a coating 5 external to the muslin tape 3. The inner coating and the outer coating of the varnish-talc mixture are indicated by the reference characters 5 and l. The various terminals 8 for the transformer are shown after the varnish and varnish-talc coatings thereon have been removed. It is of course to be understood that in the drawing the thickness of each varnish or varnish-talc coating has been somewhat exaggerated. F om the disclosure in the drawing it will be apparent that the smooth glassy-like moisture-proof coating produced by this invention extends not only around the coil of the transformer but also completely covers all metal parts of the transformer and leads, resulting in a continuous coating surrounding the whole structure.

What is claimed is:

1. .he method of forming a smooth adherent moisture-resistant coating on the external surface of an electrical device comprising a formed coil, which method comprises applying to the external surface of said device a coating of a polymerizable varnish comprising a solution of the reaction product of cashew nut shell oil and formaldehyde, drying and baking said coating at a temperature of about 276 F. for a time of about eight to ten hours, applying to said device a coating of a coating compound comprising said varnish and finely divided talc combined such relative amounts that the weight of the finely divided talc is about per cent to 9:) per cent of the weight of the solid content of the varnish, drying the coating compound on said device at room temperature for a few hours, baking the coating compound on said device for a period of about five hours at a temperature of about 190 F. to 210 subsequently applying a final coating of said coating compound to said device, drying said final coating at about room temperature for a few hours, then baking the final coating for a period of about ten to twelve hours at a temperature of about 190 F.

* to210 F.

2. The method of forming a smooth adherent moisture resistant coating on the external surface of an electrical device comprising a formed coil having a porous fabric covering which method .1 comprises vacuum impregnating and coating said device With a polymerizable varnish comprising a solution of the resinous reaction product of formaldehyde and cashew nut shell oil, drying and baking said varnish until the resin is cured to an infusible insoluble state, coating said impresent in an amount between about 80 per cent and about per cent by weight of the resin, said coating composition having a viscosity of the order of 49 seconds when measured in an A. S. T. M. cup with a .2 inch hole, drying said coating compound after each coating at a sub stantially non-curing temperature and baking said coating compound after each coating, the total baking time at the baking temperatures employed being insufficient to cure any of said additional coatings to an infusible, insoluble state but sufiicient to cure the outer coating to a flexible, solid state.

3. The method of forming a smooth, adherent, moisture resistant coating on the external surface of an electrical device comprising a formed coil, which method comprises applying to the external surface of said device a coating of a polymerizable varnish comprising a solution of the resinous reaction product of cashew nut shell oil and formaldehyde, drying and baking said varnish coating until the resin is cured to an infusible, insoluble state, applying to said coated device at least two additional coatings of a coating compound comprising a solution of said resinous reaction product mixed with finely divided talc combined in such relative amounts that the weight of the talc is between about 80 per cent and about 90 per cent of the weight of said resinous reaction product, drying each coating at a substantially non-curing temperature and baking said coating compound after each coating, the total baking time at the baking temperature employed being insuificient to cure any of said additional coatings to an infusible, insoluble state,

7 but sufiicient to cure the outermost coating to a flexible solid state.

4. The method of forming a smooth, adherent, moisture resistant coating on the external surface of an electrical device comprising a formed coil having a porous fabric covering, which method comprises vacuum impregnating and coating said device with a polymerizable varnish comprising a solution of the resinous reaction product of cashew nut shell oil and formaldehyde, drying and baking said coating at a temperature of about 270 F. for a time of about 8 to 10 hours, applying to said device a coating of a coating compound comprising said varnish and a finely divided talc combined in such re1- ative amounts that the weight of the talc is about 80 per cent to about 90 per cent of the weight of the solid content of the varnish, said coating compound having a viscosity of the order 49 seconds when measured in an A. S. T. M. cup with a .2 inch hole, drying the coating compound on said device at room temperature for a few hours, baking the coating compound on said device for a period of about five hours at a temperature of about 190 F. to about 210 F. subsequently applying a final coating of said coating compound to said device, drying said final coating at room temperature for a few hours and then baking the final coating for a period of about 10 to 12 hours at a temperature of about 190 F. to about 210 F.

ROBERT W. DE MONTE.

ATWELL B. HAINES.

REFERENCES CITED The following references are of record in the file of this patent:

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